1. Field of the Invention
The present invention is directed to a position-measuring device, and more particularly, a position-measuring device that generates a high-resolution reference pulse signal.
2. Discussion of Relevant Art
In prior-art optical position-measuring devices, a scale having both an incremental track and a reference track are provided to allow absolute positions to be determined. In order to generate an accurate absolute position the reference marks in the reference track should have as high a resolution as the incremental marks in the incremental track. Consequently it is desired, particularly for high-resolution interferential position-measuring devices, to have as narrow as possible a reference pulse. It is not possible, however, to provide such high resolution using traditional, non-interferential methods for the generation of reference pulse signals. U.S. Pat. No. 5,428,445 discloses employing so-called chirped grid graduation structures to generate high-resolution reference pulse signals in interferential measuring systems. The graduation structures needed for this have location-dependent, continuously increasing or continuously decreasing graduation periods. That is, the graduation structures do not have the same graduation period over their entire length. These types of graduation structures from the point of view of development as one proceeds from a complete grid are in turn, divided into a series of smaller subgrids with each subgrid having a constant but different grid constant. Each of the individual subgrids provides, during optical sampling, a defined contribution of differing frequency to the resulting total signal. Due to the superposition of the varying-frequency signal contributions of the different subgrids, a non-periodic signal with a sharp, spatially defined maximum results. By suitable variation of the grid parameters of the chirped grid structures the amplitudes and phase of the different signal contributions and thus the resulting signal can be selectively influenced. In particular a desired high-resolution signal for the reference pulse signal can be selected in this way.
Due to certain design requirements, for example, problems result when a reference pulse signal based on chirped graduation structures is to be generated for large distance between the scale grating and scanning gratings. Thus for large distance between the scale grating and scanning grating, between the chirped graduation structure and the scanning graduation the individual beams split by the scanning unit strike areas on the graduation that lie further removed from one another than is the case for a comparatively small distance between the scale grating and scanning grating. The same effect occurs if, at a given distance between the scale grating and scanning grating, the local graduation period is reduced, as is possibly required for a narrower reference pulse.
While at a smaller distance between the scale grating and scanning grating it can be assumed that the local grid constant of the incident differing graduation areas is approximately identical insofar as the graduation period only changes slowly, this assumption no longer applies in case of an enlarged distance between the scale grating and scanning grating or a reduction of the width of the reference pulse. Rather, at the different points of incidence of the split individual beams, different local grid constants are present. Moreover, in order to generate an interference signal, the individual beams issuing or splitting off from a point must, however, be combined again approximately at one common point. Hereby the individual beams diffracted in different directions should preferably have run through the same optical path length. This is no longer possible due to the different diffracted effect of the areas of different local grid constants for the various individual beams in the case of large distance between the scale grating and scanning gratings.
One solution may be to provide the scale chirp fields with a very slowly changing graduation period. High-resolution reference signal pulses depend, however, on correspondingly small local grid constants with a large deflecting effect. Thus large-surface sample fields result, which contradicts the requirements for a compact design of the sampling unit.
Along with the requirement for larger distance between the scale grating and scanning gratings, between the scanning and scale graduations a certain insensitivity of the generated reference pulse signals with respect to the so-called Moire rotations is desirable in high-resolution interferential position-measuring devices. Hereby let Moire rotation be understood to mean a rotation of the scale and scanning graduations about an axis perpendicular to the scale graduation. Even in the case of a rotation of this type a constant position of the reference pulse signal relative to the incremental signal must be guaranteed.
A particularly adjustment-insensitive optical position-measuring device for the registration of the position of two objects movable relative to one another is known from U.S. Pat. No. 5,079,418. The disclosed position-measuring device operates according to the interferential principle and includes, along with the scanning unit and scale graduations at least one retroreflecting element that, after the initial pass through of the scanning and scale graduations, causes a deflection of the individual beams back in the direction of incidence so that the scanning and scale graduations are passed through again. A particularly adjustment-insensitive measuring arrangement with high resolution is provided due to the retroreflection provided and the repeated passing through of the scanning and scale graduations.
It is thus an object of the present invention to generate as high-resolution reference pulse signals as possible, particularly in an interferential position-measuring device. It is also an object to provide greater distance between the scale grating and scanning gratings between the scanning graduation and the scale graduation in order thereby to achieve a greater flexibility with regard to various design requirements.